I can access the authorized directories without problems using Windows Explorer or Cyberduck.
However, WinSCP seems unable to negotiate the 401 challenge. It does not even get to the password prompt, i get two error messages with the same text:
" Authentication failed.
Could not authenticate to server: ignored Negotiate challenge, ignored NTLM challenge"
This happens on port 80 and also port 443 (SSL).
See attached log file (level Debug1).